(a) Field of the Invention
The present invention relates to a quantum-confinement Stark effect (QCSE) optical modulator, and an integrated semiconductor optical device including a semiconductor laser device and a QCSE optical modulator integrated on a single chip.
More specifically, the present invention relates to such a QCSE optical modulator or a semiconductor optical device having a lower device resistance and an excellent frequency characteristic.
The present invention also relates to a method for fabricating the QCSE optical modulator and an integrated semiconductor optical device including the QCSE optical modulator and a semiconductor laser device.
(b) Description of the Related Art
In a super-lattice structure, if an electric field is applied normal to the layers, excitons are hardly dissociated in a moderate electric field due to the presence of a barrier layer preventing the dissociation of the excitons. For example, if an electric field of 104 volts/cm is applied to a quantum well having a width of 10 nm, the quantum well is inclined by an amount corresponding to 10 meV. This range of the electric field scarcely causes the excitons in the quantum well to dissociate, and only a peak of the optical absorption spectrum is observed to shift toward a lower energy level. This phenomenon is called QCSE.
A QCSE optical modulator including an AlGaInAs-based quantum well structure and taking advantage of the QCSE is described in xe2x80x9cJournal of Lightwave Technologyxe2x80x9d, vol. 8, No. 7, July 1990, and recited to have a lower operational voltage and achieve a higher-speed modulation compared to GaInAsP-based QCSE optical modulator.
FIG. 1 shows a conventional QCSE optical modulator (may be referred to as simply xe2x80x9coptical modulatorxe2x80x9d hereinafter). The optical modulator is of a waveguide type and includes a p-i-n structure formed on an InP substrate 112, the p-i-n structure being such that AlGaInAs/AlInAs multiple quantum well (MQW) 118 constituting an intrinsic layer is sandwiched between a p-type cladding layer 120 and an n-type cladding layer 116.
The optical modulator 110 has the n-InP substrate 112 and a layer structure including an n-type InP (n-InP) layer 114, the n-AlInAs cladding layer 116, the MQW 118, the p-AlInAs cladding layer 120, and a p-InGaAs contact layer 122,. which are consecutively grown on the n-InP substrate 112 by a molecular beam epitaxy (MBE). A p-side electrode 124 and an n-side electrode 126 are formed on the p-type contact layer 122 and the bottom surface of the InP substrate 112, respectively.
The MQW 118 includes a plurality (30) of film pairs each including a 86-angstrom-thick AlGaInAs quantum well layer and a 50-angstrom-thick AlInAs barrier layer and formed in a cyclic order. The n-AlInAs cladding layer 116, MQW 118, p-AlInAs cladding layer 120 and p-InGaAs contact layer are configured as a ridge stripe of a higher mesa structure having a width of 4 xcexcm and a length of 90 to 120 xcexcm.
The p-InP layer 114 and the ridge stripe are covered by a SiO2 film except for the p-side electrode 124.
When a reverse bias voltage is applied to the conventional optical modulator 110 of FIG. 1, the QCSE shifts the peak of the optical absorption spectrum of excitons toward the longer wavelength side, thereby increasing the optical absorption effect of the optical modulator for the laser. This operation uses a reverse bias voltage of the p-i-n junction as a drive current for the change of the optical absorption, and thus achieves a larger change of the optical absorption at a high speed by using a small voltage.
In the conventional optical modulator 110 as described above, there is a problem in that the higher mesa structure of the ridge stripe generally has a rough surface formed on the ridge wall during the etching for configuring the mesa structure, the rough surface causing scattering loss of transmitted light to degrade the device characteristics.
In addition, the small ridge width of the mesa structure raises the resistance of the p-type cladding layer, which has in general a larger resistance compared to the n-cladding layer, and raises the overall device resistance.
Further, it is difficult to adopt a selective growth technique in the case of integration of the conventional optical. modulator with a semiconductor laser device, due to the presence of the Al content in the material for the MQW and the cladding layer.
For the reasons as recited above, a semiconductor optical device having an optical modulator and a semiconductor laser device integrated in a single chip generally has a higher device resistance and lower device characteristics.
In view of the above problems in the conventional techniques, it is an object of the present invention to provide an optical modulator having a lower electric resistance and excellent device characteristics, to provide a semiconductor optical device including the optical modulator and a semiconductor laser device integrated in a single chip.
It is also an object of the present invention to provide such an optical modulator and a semiconductor optical device.
The present invention provides a quantum confinement Stark effect (QCSE) optical modulator including a compound semiconductor substrate, and a layer structure formed thereon, the layer structure including an AlGaInAs-based multiple quantum well (MQW), a pair of cladding layers having opposite conductivity types and sandwiching therebetween the MQW, and an Al-containing layer overlying the MQW or formed within one of the cladding layers having a p-type conductivity, the layer structure being configured as a ridge stripe at a portion including the Al-containing layer, the Al-containing layer having a current confinement structure wherein a pair of Al-oxidized regions of the Al-containing layer sandwiches therebetween a central un-oxidized region of the Al-containing layer.
In accordance with the optical modulator of the present invention, the width of the cladding layers in the ridge stripe is wider and thus the device electric resistance is lower compared to the conventional optical device, due to the presence of the current confinement structure having a pair of Al-oxidized regions sandwiching therebetween un-oxidized region. In addition, the presence of the current confinement structure having the pair of Al-oxidized regions prevents a rough surface of the ridge side, and thus reduces the transmission loss of the laser. These advantages also result in improvement of the frequency response of the optical modulator.
The Al-containing layer may be an AlInAs layer.
The present invention also provides method for fabricating a quantum confinement Stark effect (QCSE) optical modulator including the steps of:
forming a layer structure on an InP substrate, the layer structure including a multiple quantum well (MQW), pair of cladding layers sandwiching therebetween the MQW and an Al-containing layer overlying the MQW;
configuring at least a portion of the layer structure including the Al-containing layer to form a ridge stripe; and
selectively oxidizing Al in the Al-containing layer to form a current confinement structure in the ridge structure, the current confinement structure having a pair of Al-oxidized regions of the Al-containing layer sandwiching therebetween an un-oxidized region of the Al-containing layer.
In accordance with the method of the present invention, the structure of the QCSE optical modulator optical device having a lower electric resistance and improved frequency characteristics can be formed with a simple process.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.